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S. purpurea purpurea is very hardy. There are limits. A plant set outside above ground in a pot even if it is native to much colder areas could get in trouble if a long period of very cold harsh weather exsist. In habitat natural insulation occurs from just being in the ground. Exposed in a pot the plant & roots get the full fury of cold weather. What happens a lot of times is a thaw occurs in later winter followed by another blast of severe cold. The plant can start to emerge from dormancy as the thaw exposes the plant to warmer weather than if it were in the ground where the soil would not warm up during a brief thaw.
Fungal infections commonly take hold and kill the plant.
In a cold climate, such as Chicago, Buffalo, northern Ohio it is best to either bury the pots or cover them with a pine mulch (up to the crown). It is true that S. purpurea purpurea grows farther north than this, but why risk it with a potted plant?
S. purpurea venosa is hardy too. The far southern form S. purpurea burkeii is not hardy for long term cold weather. It can take very cold shots of air for short term, but long term can often take it out.
Later,
~ Mike

the same pretty much goes for any potted plant, heat is evenly distributed by soil, in a pot, wind, rain, you name it, can strip the heat out of a pot and turn it to an ice cube in a suprising short period of time... try an experiement if you live some place cold... fill a pot of up with soil, and keep it moist. When the first extended cold blast comes through, snow falls, and fun begins, the pot will freeze up, yet you will still be able to dig into soft soil in the ground, shows you the difference...